There is a communication system in which each of a plurality of wireless terminals, sharing a carrier (a wireless channel), transmits data obtained by a sensor to a destination host. When strict demands for power saving are imposed on each wireless terminal, a relay network is provided between the plurality of wireless terminals and the destination host. The relay network is a wireless network formed by a plurality of wireless relay apparatuses. The provision of the relay network allows the coverage of radio waves from each wireless terminal to be narrowed as compared with the case without the relay network. The data transmitted from each wireless terminal reaches the destination host via one, or two or more wireless relay apparatuses.
For further information, see Japanese Laid-Open Patent Publication No. 2009-177292.
The demands for power saving, namely, demands for the reduction of power consumption may be imposed also on wireless relays. One of the factors causing power consumption in each wireless terminals and wireless relay apparatuses is a receiving process of wireless frames that each of the terminals and the relay apparatuses does not need to receive.
When a carrier is shared among a plurality of wireless terminals, each wireless relay apparatus performs carrier sense on the carrier and performs a process of receiving data in a detected wireless frame. For that reason, the wireless relay apparatuses also share the carrier with the wireless terminals. In this case, when a wireless terminal or a wireless relay apparatus exists within an area where a radio wave transmitted from each of the other wireless terminals and wireless relay apparatuses reaches, the wireless terminal or the wireless relay apparatus performs a receiving process with power consumption even when the wireless frame is not addressed to the wireless terminal or the wireless relay apparatus. In such a manner, there are some cases where needless receiving processes waste power.